I commend you for this post, sir. I've only just come across this forum and the lack of knowledge for a supposed "emo" forum atrocious, so much so that I complained in the "Top 10 Bands" thread. If you believe that any bands currently in rotation on MTV and the like are emo/screamo, I urge you to read the above post and checking out the key bands to get a good idea of the general sound/feeling/attitude of the genre. If you still require more information visit:
www.fourfa.com
http://en.wikipedia.org/wiki/Emo
These sites contain much of what Zealious said but with some added history/definitions. If anyone is still is unsure of what emo is or finds it hard to find some mp3's I'm even prepared to upload some of the key bands for you.